The problem is that ALL 320 series (318,319,320,321) share the same line numbers - so all you get to know from a casual look at the production list is that UA got the 500th A320 SERIES a/c (a 320), AF got the 1000th (a 319), TACA got the 1500th (another 320) - to find out who got the 500th or 100th A320 is rather difficult (but I´d sure like to know).
United Airlines N485UA first flight 08/10/01 del. date 12/13/01 was the 1000th A320 delivered. And it seems United took delivery of the 1001st, N486UA on the same day.
Here's the source: http://www.jetairliners.com/airbus/3204.htm Cheers.